The present invention relates to an ultrasonic motor piezoelectric actuator element.
An ultrasonic motor utilizing the resonant mode of an elastic body exhibits excellent controllability and has attracted attention particularly as a precise positioning actuator. A linear ultrasonic motor is generally required as an actuator for various stages. Therefore, various types of linear ultrasonic motors have been proposed and studied. In particular, various ultrasonic motors utilizing composite longitudinal (expansion and contraction) vibrations and bending vibrations of a rectangular plate have been studied (see, for example, Patent Document 1). Among them, for example, an ultrasonic motor having a principle disclosed in the Patent Document 1 has been put to practical use for various applications, wherein a rectangular piezoelectric element plate is polarized in its thickness direction, four divided electrodes are provided on the piezoelectric element plate, and a vibrator formed of the piezoelectric element is caused to produce longitudinal vibrations and bending vibrations by applying a driving signal to a pair of electrodes located on the diagonal line of the vibrator to drive a moving body in contact with the vibrator.
The Non-patent Document 1 discloses an ultrasonic motor element using a primary longitudinal mode and a secondary bending mode and has a description of a piezoelectric actuator element where rectangular piezoelectric layers and electrode layers are stacked alternately in such a manner that the electrode layers divide the piezoelectric layers into two approximately equal portions.
FIGS. 13(a) and 13(b) are schematic views showing this piezoelectric actuator element. The piezoelectric actuator element has a structure in which piezoelectric layers 10 and electrode layers 12 are stacked alternately. This piezoelectric actuator element operates similarly to the piezoelectric element having the structure disclosed in the Patent Document 1. Specifically, a vibrator formed of a piezoelectric element is caused to produce longitudinal vibrations and bending vibrations to drive a member in contact with the vibrator.
[Patent Document 1] Japanese Patent No. 2980541 Specification Non-patent Document 1 Proceedings for Actuator 2006 A1.1 (Piezoelectric Ultrasonic Motors for Lens Positioning of Cellular Phone Camera Modules).
However, the structure disclosed in the Patent Document 1 requires an extremely high voltage to drive the vibrator, whereby a large voltage booster circuit is necessary. This results in an increase in size and complexity of a driver circuit, thereby increasing the cost of the driver circuit. This makes it difficult to apply the structure disclosed in the Patent Document 1 to small-sized instruments (particularly instruments driven by a battery). If the thickness of the piezoelectric element (vibrator) is small, a sufficient output cannot be obtained. Moreover, the piezoelectric element may break due to low mechanical strength.
The ultrasonic motor element disclosed in the Non patent Document 1 contributes to reducing the size of the driver circuit as compared with the structure disclosed in the Patent Document 1. However, since degree of sintering of the center portion of the element is poor after sintering, migration tends to occur. Moreover, when the element is subjected to alternating polarization in order to achieve highly efficient polarization, the amount of heat generated in the element increases, and thereby cracks tend to occur in the center portion of the element.
In this ultrasonic motor element, since the electrodes are located at an identical position in the projection plane of the element (i.e. the electrodes are provided at an identical distance), the stacking pressure is applied to the piezoelectric regions to only a small extent near the edge of the electrode, whereby inferior stacking or delamination tends to occur.